produce the Permanent and Periodic Winds. 



445 



Thus, then, according to the best evidence that can be produced, 

 the theory already laid down is at variance with observation ; and 

 we are led to ask why should the Atlantic air from the latitude 

 of 30° N., at a temperature which in summer may be put at 72° 

 or 77°, rush southward to where the thermometer stands at 8.2°, 

 instead of eastward to where the thermometer ranges about 100°. 

 The aqueous vapour of the oceanic air is not sufficient to account 

 for this, as is shown by the following Table of densities calcu- 

 lated approximately, on the supposition that the dew-point is 5° 

 below the temperature of the air : — 



Density of air. 



Temperature 

 (Fahr.). 



Moist. 



Dry. 



•9184 

 •9086 



•8987 



1-0000 

 •9259 

 •9174 

 •9091 

 •9009 

 •8928 

 •8849 

 •8772 

 •8695 

 •8620 



32 



72 



77 



82 



87 



92 



97 



102 



107 



112 



The dew-point in the equatorial oceanic district is perhaps 

 seldom so much as 5° below the temperature ; Professor Daniell 

 has estimated the difference as generally 2° or 3° ; but, on the 

 other hand, the desert air is by no means perfectly dry, so that 

 there is probably very little error in the difference of densities as 

 shown above. Now, independently of the evidence of the Pilot- 

 Charts, no fact in meteorology is better known than the frequency 

 of hot easterly winds on the northern part of the west coast oi 

 Africa : these winds, charged with sand from the desert, come 

 off dry and scorching, streaming into a denser air, in apparent con- 

 tradiction of the hydrostatic law. 



Captain Maury considers that the rarefaction of the desert 

 air makes itself manifest in the " African monsoons of the At- 

 lantic;" but these winds (which I have shown in the last two 

 columns of the foregoing Table) are not experienced for some 

 degrees south of the southern border of the Sahara, w T hich 

 scarcely extends beyond the seventeenth parallel of latitude; and 

 we cannot accept the conclusion that the Sahara will draw in the 

 more distant and less dense air of the equator in preference to 

 the neighbouring and denser air between the parallels of 20 

 and 30° N. 



